I've had 12" PowerBook, 15" MBP, 13" MBA and i would not endure a 10" widescreen tablet/subnotebook.
Anyhow - apple is not releasing anything for a niche. It will be for the mass market. And it really has to have a keyboard. There is no point in releasing an increased size iphone/touch-brother without a keyboard. that is the only shortcoming for a productivity tool.
An onscreen keyboard makes more and more sense the bigger the screen gets. You get more accurate typing and less obstruction of elements on the screen. There is as yet no indication whether this tablet will be media oriented or productivity oriented. It would make sense that it would be media oriented (e-books, on-demand movies, AppleTV functionality), as the market would possibly be larger. There are already productivity oriented tablets around such as the modbook, and they do serve only a tiny niche.